Transition-metal-doped gain-media such as Ti:sapphire (Ti:Al2O3), Alexandrite (Cr:BeAl2O3), Cr:LISAF (Cr:LiSrAlF6), Cr:LICAF (Cr:LiCaAlF6), Forsterite (Cr:MgSiO4) chromium-doped yttrium aluminum garnet (Cr:YAG) and chromium-doped zinc selenide (Cr:ZnSe) provide large wavelength tuning ranges of several hundred nanometers (nm) with relatively large cross-sections for stimulated emission. This enables ultrafast pulse generation with sub-picosecond (ps) pulse durations, down to less than ten femtoseconds (fs)
The most prominent of these transition-metal-doped gain-media is Ti:sapphire, which in the last two decades has found widespread use as a gain-medium in commercial ultrafast solid-state lasers. Titanium exhibits a broad tuning range of the emission wavelength between about 650 nm and about 1080 nm, and a relatively large emission cross-section of about 2.8×10−19 cm2 at the peak-emission wavelength. To date commercial Kerr lens mode-locked Ti:sapphire oscillators provide average output powers of up to 3 Watts (W) at a pulse-repetition frequency (PRF) of about 80 MHz, with typical pulse durations between about 10 and 150 fs. Pulse amplification in Ti:sapphire amplifiers enables pulse energies of up to 15 millijoules (mJ) per pulse at typical repetition rates of between about 1 and 5 kilohertz (kHz).
The gain-medium in all commercial Ti:sapphire lasers and amplifiers is in the form of a rod or slab that is end-pumped by continuous-wave (CW) green (frequency doubled) solid-state lasers with up to 18 W of pump power, or by Q-switched frequency-doubled Nd:YAG and Nd:YLF lasers emitting pulse energies of up to 50 mJ at 527 nm or 532 nm with pulse durations around 100 ns, PRF up to 10 kHz, and average output powers up to 100 W. FIG. 1 is a graph summarizing the performance of mode-locked Ti:sapphire oscillators and amplifiers for different pump power levels. The oscillator data is all at 80 MHz in the lower right of the graph.
Ti:sapphire has a large saturation intensity of 160 kilowatts per square centimeter (kW/cm2). This is a result of the short excited-state lifetime of 3.2 microseconds (μs). Because of this, the pump-laser beam has to be focused very tightly to generate enough gain. In general, the product of pump-intensity times the ratio of pump-wavelength to laser-wavelength (this ratio typically is between 0.66 and 0.9) has to equal the saturation intensity of the gain-medium to achieve a small-signal gain g0l of 1.0.
For many transition-metal-doped gain-media, the pump-intensity has to be greater than 100 kW/cm2 in order to have high enough small-signal gain. In mode-locked Ti:sapphire oscillators, for example, the pump-spot diameter is typically less than 50 micrometers (mm) to achieve a pump-intensity of 200 kW/cm2. This results in strong thermal lensing (up to 200 Diopters) in the gain-medium, and limited power-scaling capability in TEM00 mode operation due to pump-induced thermal aberration. In Ti:sapphire amplifiers, the use of Q-switched green pump-lasers with relatively high pulse energies and pulse intensities allows the increase of the pump-spot diameter to about 1 mm. However, due to a relatively large quantum defect of Ti:sapphire (about 35%) heat generation also limits the power scaling of Ti:sapphire amplifiers.
At present, the maximum average power than can be extracted from a standard Ti:sapphire amplifier is around 15 W. The current technique to further power scale Ti:sapphire amplifiers is to use several amplifiers in series or to apply cryogenic cooling to the Ti:sapphire gain-medium. With liquid-nitrogen cooling, the thermal conductivity can be increased by one order of magnitude and the temperature derivative of the refractive index (dn/dT) is decreased by almost one order of magnitude, resulting in a 50-times decrease of the thermal lensing and a similar decrease of thermal aberration. Cryogenically cooled Ti:sapphire amplifiers have generated up to 25 mJ of pulse energy at 1 kHz, but the cooling equipment adds considerable cost and footprint to the laser system.
There is a need for a more effective Ti:sapphire amplifier architecture that allows improved power scaling without the need for incorporating non-standard cooling techniques. Very similar power-scaling limitations exist for other transition-metal-doped gain-media that have high saturation intensities, such as Cr:BeAl2O3, Cr:LISAF, or Cr:Forsterite, or exhibit poor thermal-lensing properties, like Cr:ZnSe.
It is well know that a thin-disk geometry for a gain-medium provides excellent thermal management due to a small thickness of the disk (of around 150 microns) and a large cooling area in contact with a heat-sink. Thermal management can be further increased by using heat-spreader material, for example, diamond, between the disk and the heat-sink. For high power Yb:YAG thin-disk lasers having a saturation intensity of about 10 kW/cm2, with typical disk thicknesses of between about 100 and 200 μm and pump-spot diameters between 4 and 10 mm, output powers of greater than 5 kW per disk have been demonstrated for pump intensities of up to 20 kW/cm2 and heat generation in the disk of up to 2 kW/cm2. However, the scientific literature clearly states that the thin-disk concept is not well suited for Ti:sapphire gain material or Chromium doped LISAF, because of the above-discussed high saturation intensity in combination with a large quantum defect and poor thermal properties (in case of LISAF).
This statement is certainly true if a pump-laser in CW operation is used. For Ti:sapphire, and an absorbed CW pump-power of 50 W at 532 nm, the pump-spot diameter has to be less than 500 μm in order to generate to generate sufficient small-signal gain (g0l=0.2) with each incidence of a pump-beam on the disk. The generated heat per cooling area of about 8 kW/cm2 is a factor of four to five times higher than what is thermally manageable for current thin-disk technology. In addition, a low aspect ratio (pump-spot diameter to thickness) results in temperature gradients perpendicular to the beam propagation direction which further limits the ability to generate a high power TEM00 mode beam.
In order to provide a power scalable Ti:sapphire oscillator and amplifier concept, it will be necessary to overcome the thermal limitations of the current gain-geometries and find design criteria for a thin-disk Ti:sapphire gain-medium that offer improved thermal management without decreasing the gain in the gain-medium. These thin-disk design criteria should also be applicable to other transition-metal-doped gain-media discussed above. All of these gain-media have high saturation intensities, and most have high quantum defects and poor thermal properties which, without proper pumping and cooling designs, make them unsuitable for a thin-disk gain-medium approach.